The invention relates to a charge-pump circuit intended for use in a frequency control loop of a frequency synthesizer provided with a local oscillator and a reference oscillator, said charge-pump circuit comprising an integration capacitor, a controlled output pull-up current-source and a controlled output pull-down current source.
The invention also relates to an integrated circuit comprising a charge-pump circuit.
The invention further relates to a radiowave receiver comprising a synthesizer with a charge pump, for example, a radio or television receiver, or a radio-telephone (all types: GSM, DECT, CT2, etc.).
A charge-pump circuit as described in the opening paragraph is known from the document U.S. Pat. No. 5,184,028 (Don W. Zobel).
The frequency control loop of a frequency synthesizer comprises a charge pump which serves in known manner for pull-up or pull-down of a capacitor whenever it detects a phase error in one sense or another between the local oscillator signal and the reference oscillator signal. The charge pump is thus used for compensating phase deviations. Its operation is critical with a view to noise, because the error signal is used as frequency reference for processing the useful signal.
Ideally, the current sources of the charge pump supply current pulses of a very short duration, for example, of the order of several nanoseconds, of a controlled amplitude and a great spectral purity. If the result of the summation of the two pulses with an equal amplitude but an opposite polarity is zero, this is not equally the case for their summation in terms of noise. Certain noise contributions are correlated between the two sources and their sum is zero. Other parts are due to independent noise mechanisms whose noise values are added. This residue limits the quality of the signal from the oscillator.